Support subscription plan changes in Core API

🔍 Problem Statement
Change plan functionality does not exist in the Core API, which forces teams to rely on manual intervention.

💡 User Story
As a developer, I want the Core API to support change plan functionality so that I can manage subscription plan changes programmatically in bulk, without manual steps, and maintain consistency with the platform and Open API.

🎯 Definition of Done (DoD)

A feature is considered complete when:

✔️ The Core API subscriptionUpdate endpoint supports change plan functionality.
✔️ The Core API subscriptionUpdate endpoint supports updating address functionality.
✔️ All validations match those that exist in the platform/Open API.
✔️ Subscriptions can be updated via the Core API without requiring manual intervention.

Test Cases

• Changing a subscription to a plan that costs less than the next upcoming phase

• Changing a subscription to a plan that costs less than the next upcoming phase

• Changing a subscription to a plan that costs more than the next upcoming phase

• Changing a subscription to a plan that costs more than the next upcoming phase

• Changing a subscription to a plan that costs less than the next upcoming phase

• Changing a subscription to a plan that costs more than the next upcoming phase

• Changing a subscription to a plan that costs less than the next upcoming phase

• When the address ID belongs to another customer or is invalid, the error message should be changed from "This product is not available in your area"

• Trying to change an expired subscription

• Changing to an archived plan

• Changing to a plan that exists in another account should be blocked

• Changing a redeemed gift should be blocked

• Sending a subscription ID that belongs to another site

• Coupon gets applied to the new subscription invoice although it was restricted for the first plan only

• Changing from an auto-renewal plan to a one-time plan should set the subscription to cancel_at_period_end

• Changing from a one-time plan to an auto-renewal plan should set the subscription to renewal cycles instead of expire_at_period_end

• Downgrading/upgrading any subscription that has a schedule will not change the end_date for the current phase

• Changing a subscription that has future phases changes the collection method to "paying manually" on the cancelled plan and the new one